We laughed, wept and nodded along knowingly with a film that documents the suffering of asylum seekers in mandatory detention and confronts the unchallenged racism towards Muslims and asylum seekers that exists in pockets of Australian society. Heather Kirkpatrick's documentary feature was moving and powerful, but it didn't change any minds in that audience – as the filmmaker acknowledges: "People often say, 'well, you are just preaching to the converted.'"

From the end of this week, the documentary will be shown in inner-city Palace Cinemas across Australia. But since an extended season in Hobart in June, Kirkpatrick has also taken Mary Meets Mohammad around the country. She spent her inheritance to produce the debut film. Then furiously raised funds in order to show it in places where its message would meet the most cultural resistance. It has now screened at more than 80 locations, including sell-out shows on Christmas Island.

"So many people came to see it, including a lot of the Serco [detention centre] guards, that they hosted another one. It think that was great because I think they're a pretty divided community," she said.

The documentary follows the unlikely love that develops between Tasmanian pensioner Mary and Mohammad, an Afghan man imprisoned in the Pontville detention centre near her home. As their relationship develops, Mary moves from suspicion, to sympathy, to grandmotherly adoration for a Muslim man she says she would once have described as a heathen and a coward.

"We haven't been taught to think of them as human beings," Kirkpatrick says. "In Australia they're boat people, they're troublemakers. Unfortunately, we don't have media to balance that by visiting an asylum seeker in detention and getting their side of the story."

When Mary meets the man on the other side of the razor wire, or news report, her transformation is radical. Kirkpatrick says audiences across Australia have been receptive to the film and that many of them spoke to her of a shift in their attitude towards asylum seekers. "It's definitely working. I'm confident of that," she says. Speaking to people after screenings, Kirkpatrick realised that "they sort of did take that same journey as Mary".

The film's power lies partly in the testimony of Mohammad, who speaks of the tragedy of his Hazara people, forced from their homeland by the Taliban and persecuted in camps in Pakistan. He talks about longing to see his family and his struggle with depression in the face of indefinite internment.

An Australian Government report in 2012 references research that shows "clinically significant symptoms of depression were present in 86% of detainees" with "approximately one quarter reporting suicidal thoughts".

Kirkpatrick said that after meeting many asylum seekers through her work with the film, she was yet to come across one who had been through detention without becoming depressed or considering killing themselves.

But even more than Mohammad, Mary is the key to the film's appeal beyond an audience who share its concerns. The funniest moment of the film comes when she jumps on her mobility scooter to pick up a slab from the pub. In the same way as The Castle's Darryl Kerrigan, Mary is recognisable across Australian society's great divide – the flannelette curtain as we call it in Hobart.

When audiences get to see one of their own having a change of heart, Kirkpatrick says, "all those barriers just come crashing down".